1. Field of the Invention
The present invention relates to aluminum supports for lithographic printing plates and base plates for lithographic printing plates using the same. In particular, it relates to the aluminum supports suitable for so-called direct plate-making positive-type lithographic printing plates, which can be used for direct plate-making by receiving digital signals from a computer or the like.
2. Description of Related Art
Conventionally, positive-type photosensitive lithographic printing plates have a photosensitive layer component mainly comprising an O-quinone diazide compound and a binder resin such as a novolac resin, wherein the exposure of ultraviolet light leads to photodecomposition at the exposure areas and increase of solubility to developing liquids, and utilizing these characteristics, non-image areas are formed by removing the exposed areas with a developer, and non-exposed areas form the image area. These plates are commonly referred to as conventional PS plates.
Furthermore, lithographic printing plates utilizing a direct plate-making method (thermal positive) and the like have been developed, that have a photosensitive layer component mainly comprising a photothermal conversion material such as an infrared absorbing pigment, which absorbs infrared light and converts it to heat, and a binder resin such as novolac resin, wherein images are formed by altering the structure of the resin by using infrared laser exposure, and increasing solubility to developing liquids.
Regarding such lithographic printing plate materials that use infrared lasers as their exposure light source, since the energy of the infrared light used for writing is lower than that of the ultraviolet light used for conventional exposure light sources, in comparison with conventional PS plates, the gap in the speed of dissolution between exposed areas and non-exposed areas is narrower, and changes in usage conditions often cause over-exposure and development deficiencies. Also, it is difficult to dissolve those parts of the photosensitive layer that have sunk into the pores of the anodic oxidation coating of the anodized aluminum support, and this results in scumming and other problems.
In this regard, for example, Japanese Patent Application Unexamined Publication No. H10-297130 A discloses an aluminum support for lithographic printing plates having particles of 5 μm or smaller on its surface, but although such a support is effective for conventional PS plates, this sometimes makes it easy for problems such as scumming to occur with thermal positive plates, which have a small development latitude.
Furthermore, Japanese Patent Application Unexamined Publication No. 2002-116549 A teaches a base plate for a lithographic printing plate having on a support with a coating layer containing an inorganic fluorine compound and a phosphorus compound, a photosensitive layer containing a high molecular compound whose solubility in alkaline solutions is varied by heat and a material which absorbs light and generates heat. However, there are problems with such supports in regard to a photosensitive layer containing a photoacid generator and an acid-decomposable compound, such as a tendency for portions of the film of the photosensitive layer to become residual, as well as worsening of chemical resistance against dampening solutions, plate cleaners and the like, and lower print durability.